Stretched polypropylene films are widely used in various fields such as food wrapping and fiber wrapping because of their good optical properties (e.g., transparency and gloss), mechanical properties (e.g., tensile strength and stiffness), and moisture proofness.
Single layer-type stretched polypropylene films, however, are very poor in heat sealability. Therefore, if the films are heat sealed at temperatures at which they are heat sealable, heat shrinkage occurs and they cannot be practically heat sealed.
Various attempts have been made to overcome the above problem, including a method in which only adhesion areas are partly coated on a stretched polypropylene film as a base layer, a method in which an easily heat sealable resin is coated, a method in which another film is laminated on a stretched polypropylene film through an adhesive, and a method in which an easily heat sealable resin is laminated on a stretched polypropylene film by melt extrusion. Japanese Patent Publication No. 11353/66, for example, discloses a method in which an ethylene polymer having good heat sealability, such as an ethylene/vinyl acetate copolymer and a low or intermediate density polyethylene, is laminated in the film form on at least one side of a stretched polypropylene film. It is known, however, that lamination of such an ethylene polymer gives rise to various problems particularly in scratch resistance.
Japanese Patent Publication Nos. 31478/71 and 14343/74 disclose a method of producing a film having good scratch resistance and also relatively good heat sealability, in which a crystalline propylene/ethylene copolymer is laminated. It is also known that of crystalline propylene/ethylene copolymers, a propylene/ethylene random copolymer is relatively good in transparency and has good heat sealability. The heat sealable temperature of the propylene/ethylene random copolymer drops as the ethylene content is increased. In order to increase the heat sealability of the stretched composite film, therefore, it is desirable to increase the ethylene content as much as possible.
As the ethylene content of the propylene/ethylene random copolymer is increased, the scratch resistance of the film is reduced and, furthermore, antiblocking and sliding properties, which are important properties for films, are deteriorated. In general, therefore, an antiblocking agent, such as silica, is added to prevent the deterioration of antiblocking properties. The addition of such antiblocking agents, however, inhibits the optical properties such as transparency and gloss of the film, and hence, it results in deterioration of inherent optical properties of a biaxially stretched polypropylene film. Thus the heat sealability is limited in its improvement.
In order to improve the heat sealability of the propylene/ethylene random copolymer, it may be considered to compound the above-described ethylene/vinyl acetate copolymer or low or intermediate density polyethylene, for example. However, the compatibility of the propylene/ethylene random copolymer with the ethylene copolymer is poor. Thus if the ethylene copolymer is compounded even in a small amount, the transparency of the film is seriously deteriorated.